Radio frequency or microwave (hereinafter “RF”) plasma generation equipment is widely used in semiconductor and industrial plasma processing. Plasma processing supports a wide variety of applications, including etching of materials from substrates, deposition of materials onto substrates, cleaning of substrate surfaces, and modification of substrate surfaces. The frequency and power levels employed vary widely, from about 10 kHz to 2.45 GHz and from a few Watts to as much as 100 kW or greater. For semiconductor processing applications, the range of frequencies and powers presently used in plasma processing equipment is somewhat narrower, ranging from about 10 KHz to 2.45 GHz and 10 W to 30 kW, respectively.
Plasma processing equipment typically requires a precision RF signal generator, a matching network, cabling, and metrology equipment. In addition, precision instrumentation is usually required to control the actual power reaching the plasma. The impedance of loads associated with a plasma can vary considerably in response to variations in gas recipe, plasma density, delivered RF power, pressure and other parameters.
An RF supply, including a signal generator and matching network, can deliver power to the plasma in a number of ways, for example, via an antenna or sample holder. An antenna typically has a primarily inductive load impedance, with a smaller resistive component. In contrast, a sample holder (a “chuck” or “bias”) typically presents a primarily capacitive impedance, also with a smaller resistive component.
Matching networks are typically positioned between the output of the RF generator and the input of the process chamber. The matching network provides a means of matching the output impedance of the generator to the input impedance of the process chamber. A matching network often includes elements such as variable capacitors and variable inductors to permit dynamic impedance matching of an RF generator to a changing load.
Most RF generators for plasma processing equipment are designed to have a standard fifty-ohm output impedance. A matching network can accommodate mismatches in impedance between the standard fifty-ohm output impedance of the RF generator and the input of the load. The mismatch can be exacerbated by a process chamber and plasma whose associated load can fluctuate over a large range of values.
The impedance mismatch can cause inefficient power deliver. The mismatch can also cause the power delivered to the plasma to vary, which can cause process inconsistency both within a chamber for successive substrates and among similar chambers. Thus, use of an impedance matching network can improve the efficiency of power transfer from a signal generator to a plasma vessel.
Components used in some plasma generation systems can present further difficulties in process characterization and control. For example, many systems utilize coaxial cables to connect an RF generator to an impedance matching network.
Determination of the power delivered to a reactive load (i.e., as presented by the plasma vessel) can be difficult and of limited accuracy. A standard operating method entails holding power delivered to the plasma vessel load constant. The power, however, is generally not well known because, for example, power is lost in the matchbox; the lost power is a complicated function of, for example, the positions of the vacuum variable capacitors in a matchbox plus a plasma vessel load having a nonlinear behavior.
An impedance probe can be placed between the matchbox and the plasma vessel to obtain a measure of power delivered to the plasma vessel. This approach has at least two disadvantages. First, an impedance probe can be very inaccurate when the phase angle between current voltage waveforms is high. A high phase angle typically is encountered for a highly reactive plasma vessel load. Second, impedance probes are typically too costly for production systems.